A projectile loaded into the barrel bore of a conventional air arm is propelled by the pressure of air which increases abruptly in the conduit leading to the breech as the piston is spring driven on release of the piston shaft. The energy stored in the spring is largely converted to work of compression performed as a nearly adiabatic process compressing the air ahead of the piston, but significantly large losses arise in the process of transferring kinetic energy of motion to the projectile from the compressed fluid.
Ideally, the projectile should be a body whose configuration forms a perfect gas seal in the barrel and provides a predetermined large initial resistance to being dislodged from rest in its initial, loaded position, which resistance should abruptly fall to zero once the breech pressure has reached a high value nearly equaling the peak gas pressure achieved during the piston stroke. Stated otherwise, the body should move without friction once the gas temperature has peaked, and should accelerate to maximum muzzle velocity while the breech pressure remains near or at its highest value.
In any practical barrel form the space behind the projectile is a storage vessel in which volume the entire compressed air charge is confined when the piston has been driven almost to the breech, at which time the projectile is about to be expelled. Consequently, the friction of the work of compression represented by the column of highly compressed air in the bore is not available for further acceleration of the projectile as a secondary piston. Accordingly, it will be seen that any improvement in muzzle velocity is to be attained only by avoiding losses occurring while the projectile is in the barrel.
Previously, a large number of projectile designs have been experimented with in attempting to increase the muzzle velocity. Current high-performance pellets are of “daibolo” form, i.e. they have a head portion of normal bore diameter, a reduced-diameter waist, and a flaring skirt after-portion comprising a hollow frusto-conical shell wall merging at its lesser diameter end with the head portion. Certain high-power air arms having precision rifled steel barrels are capable of accelerating the better projectile forms to muzzle velocities in the 680–780 feet per second range, excluding compression-ignition assist by ether or a hydrocarbon vapor released from the pellet.
The known forms of pellets are made from such materials and have their dimensions so chosen as to provide adequate frictional holding in the breech end of the barrel so that the inserted pellet will reliably remain stationary during barrel-closing and sighting, and so that a certain amount of drag resistance to movement is provided until the pressure has risen to several hundred pounds per square inch. Once this pressure is reached the forward movement of the pellet “grooves” the largest-diameter surface portions, which engage the barrel lands and the rifling grooves, thereby imparting rotary movement to the pellet so that it is expelled from the barrel with a high spin velocity advantageous to trajectory stability. Lead and lead alloys are cast or molded in dies to produce such pellets, after which careful selection and cushioned packaging are performed to ensure that the pellets are without deformation when they are to be fired.
In general, the best prior art pellets have a skirt margin which is relatively stiff and unyielding, so that insertion into the breech end of the barrel, even when the breech is tapered, requires firm pressure by the user's thumb to perform the initial swaging operation. While firm seating is achieved by such pellet forms, and a certain amount of initial build-up of gas pressure in the breech chamber is assured before the pellet breaks away from the static friction restraint, the pellet is not inherently self-aligning with the barrel axis, nor is the periphery of the skirt capable of being urged into such intimate engagement with the barrel bore and the rifling grooves as to avoid substantial “blow-by” of compressed air.
Certain air arms have a pellet-receiving breech and portion of the barrel wherein the bore has a diameter nearly equal to the diameter as measured between opposed rifling grooves, or even slightly larger than this diameter, so that the pellet skirt is insertable without appreciable swaging of the metal while the head portion is received in a normal bore diameter barrel portion.
While the pellet may be inserted in an initially coaxial relation to the barrel axis, as the pellet is driven forward the skirt is abruptly frictionally engaged by the reduced diameter barrel portion and remains briefly arrested until increasing air pressure drives it ahead, swaging the skirt periphery to form grooves. During this time the sealing action is imperfect which allows significantly large gas blow-by to occur, and further escape continues past the skirt margin throughout the pellet travel through the barrel.
U.S. Pat. No. 4,005,660 to J. Pichard describes a high velocity pellet for an air gun wherein the free marginal portion of the frusto-conical skirt has an inner rearward surface portion formed with a coaxial bevel so that the skirt margin tapers in thickness toward a thin rearward edge, and has a short axial length portion not longer than the internally beveled portion which flares rearwardly outwardly with a greater apical angle, the maximum diameter of the skirt periphery being so dimensioned that it is a light interference fit in a barrel diameter equal to the diameter measured across opposed rifling grooves.
In Pichard, the pellet is formed of a material such as lead or lead alloy preferably without hardening components and preferably a virgin metal which will swedge readily in its reduced thickness terminal region, enabling the skirt periphery to rapidly engage the barrel wall intimately upon rise of air pressure in the breech. When the pressure in the bore has reached its maximum value, which in well designed air arms occurs when the projectile moved only a few inches, the pellet is a freely-sliding but closely-fitted secondary piston, the skirt margin being molded to an axially-short annular ring portion of substantial constant axial length, the outer surface of which is engaged in close conformity to the transverse cross-sectional internal surface of the barrel, i.e. sliding along both the barrel lands and the bottoms of the rifling grooves. The remainder of the terminal frusto-conical portion is out of contact with the barrel. The relatively pliant terminal edge portion assures that the pellet axis coincides substantially with the barrel axis.
U.S. Pat. No. 5,150,909 to Fitzwater describes an air gun pellet comprising a spherical projectile removably retained on a skirt assembly, wherein the skirt assembly provides an arrangement for separating the projectile from the skirt assembly after the initial firing of the gun but before the projectile exits the barrel of the gun. In one version, the skirt assembly has a skirt body, with a shaft affixed to the skirt body. A projectile clutch assembly includes a clutch body and at least two clutch jaws disposed about the projectile. A retained device is disposed within the clutch body such that the projectile is retained within the clutch jaws. A conduit is disposed within the clutch body such that the shaft is capable of traversing through the conduit and propelling the projectile from the clutch jaws.
The Fitzwater device has a number of shortcomings. The release of the projectile from the skirt portion while within the barrel will result in appreciable loss of range due to premature blow-by of the compressed air around the projectile, which has a substantially smaller diameter than the internal diameter of the gun barrel. Additionally, the loss of contact of the pellet with rifling within the bore will adversely affect both range and accuracy, as will the round shape of the pellet. The multi-part structure of the Fitzwater device is expensive and prone to inadvertently separating prior to firing, resulting to jamming and mis-feeding of pellets within the air gun mechanism.
U.S. Pat. No. 4,251,079 to Earl et al. describes a pellet for an air gun which has a head portion made of metal or metal containing plastics material and a shank extending rearwardly from the head portion. A skirt portion is secured to the head portion by the shank. The skirt portion has at least two sections, which are larger in diameter than the head portion and is made of elastic plastic material, for slidably engaging the gun barrel bore surface. The head portion provides weight for the skirt portion during flight. As in the case of Fitzwater, the Earl device employs plastic to affect a seal of the compressed gasses during firing. The use of elastic materials such as plastic can result in blow-by, with resulting loss of range and accuracy. Furthermore the irregular shape of the Earl device will result in an irregular flight trajectory, with further loss of range and accuracy.
U.S. Pat. No. 6,526,893 B2 to May et al. describes polymer ballistic tip pellets including soft lead pellets with hard polymeric tips for use in air guns. The lead pellets have forwarded pointed tip portions made from a hard polymeric material. The tip portions can have hollow or solid heads. The hard tip in each of the pellets enables the pellet when fired from an air gun to pierce the fur and skin of small game animals, for example, before the lead portions of the head and skirt of the pellet begin to deform, imparting shock to the surrounding soft tissue, and shattering bone. Although the lead portion of the May design may operate in a similar manner as described in connection with the Pichard device, the two-piece design adds cost and complexity to the manufacturing process. Furthermore, any irregularities or misalignment of the lead portion and tip will result in an irregular trajectory with attendant loss of accuracy.
U.S. Pat. No. 3,649,020 to Hall describes an air gun projectile including a conventional air gun slug having a forward nose portion and a skirt portion flaring outwardly and rearwardly from a reduced diameter central portion. The nose portion is placed within the cylindrical bore of an impact-yielding cap. The cap has a circular front wall end and a rearwardly extending cylindrical skirt. The cap skirt is snugly received over the slug nose portion and the external diameter of the cap skirt is substantially equal to the diameter of the slug skirt at its widest point. Disposed within a hollow defined by the slug nose, cap front wall, and cap skirt is an indicator comprising a flash producing powder and Amorce mixture, and/or a solvent-based paint. Regarding trajectory and accuracy, the Hall device has many of the shortcomings of the other multi-part air gun pellets described herein above.
In summary, prior art pellets for air guns typically fall into two broad categories, those intended for target or non-lethal usage which have a blunt, non-penetrating head profile and remain largely intact after impact, and pellets useful for hunting which either have a projecting leading surface for high target penetration and/or features to produce substantial radial expansion or fragmentation upon contacting a target to maximize impact and soft tissue damage.
Enhancing projectile flight trajectory, and thus, range and accuracy, were immeasurably improved with the advent of rifled bores. The spiral grooves within the bore of the weapon initially impart a rotary motion component to the projectile as it accelerates down the barrel. Once discharged from the barrel, however, there is nothing other than inertia to sustain the rotary motion and it will be reduced somewhat over time due to air turbulence and drag. Thus, as it nears its impact point, its accuracy will be diminished.
To optimize flight characteristics of a projectile throughout its entire trajectory to the point of impact, means must be provided to sustain or even increase its rate of rotation. This is well known and practiced in gravity bombs and large caliber munitions, particularly those, which are chemically propelled in flight to enhance their range, such as rocket-propelled grenades and the like. Most typically, an empennage structure is added, including guide surfaces behind the projectile.
Axially directed through passages have been proposed in large caliber munitions for guidance purposes. These have the advantage of permitting internally disposed (and thus relatively protected) guide vanes to enhance rotation. U.S. Pat. No. 517,560 to Ashley describes a large caliber, armor-piercing projectile for smooth bore weapons having a central passage and internal spiral ribs, thought to enhance rotation in flight. Although the Ashley device may have utility for large caliber weapons, it has several shortcomings that render it inapplicable for smaller caliber applications. The relatively large passageway of the Ashley device results in the mass of the device being distributed externally, far from the central axis. Thus, any irregularities or asymmetries will divert the projectile from its intended trajectory. Secondly, and more importantly, simply reducing the scale of the Ashley device to a diameter typically employed in air arms would render any spiral enhancing effect negligible and probably decrease accuracy.
Although accuracy, range and flight/trajectory characteristics are desirable for any projectile, the aerodynamics of known prior art pellets for air guns is uniformly poor. This is believed to be largely due to the relatively small caliber sizes involved, and the fact that air arms tend to be in the lower end market and, thus, their consumables are extremely cost sensitive. Little or no thought has been given to streamlining the pellet profile to maximize its in-flight aerodynamic behavior. Most known designs have blunt leading surfaces and/or irregular surface features and abrupt contours which produce turbulence in the adjacent air stream and range reducing drag.
Another shortcoming of known pellets for air arms results from lack of rigorous design methodologies in optimizing their ballistic characteristics. Most pellet designs comprise nothing more that an inartistic cylindrical lump of lead with a short conical tail. Little or no thought has been given to precise placement of the center of gravity of the projectile as an essential element of maximizing in flight stability, and thus accuracy and shot-to-shot repeatability.
It is, therefore, a primary object of the present invention to provide an improved aerodynamic projectile for an air arm which overcomes known shortfalls of existing devices without adding to part count, manufacturing complexity or cost.